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1. Global Positioning
System

2. Global Positioning
System
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The basic principle of the Global Positioning System (GPS)
GPS System configuration
GPS frequencies that are used
Dilution of precision (DOP)
The various errors of GPS
Differential GPS
What is W GS 84 Datum
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• The advantages and limitation of GPS

3. • GPS satellites circle the earth twice a day in a very
precise orbit and transmit signal information to earth.
• GPS receiver compares the time a signal was
transmitted by a satellite with the time it was received.
The time difference tells the GPS receiver how far away
(distance) the satellite is.
• With distance measurements from
a few more satellites, the receiver
can determine the user’s position
and display it as a latitude and
longitude.
The basic principle of the Global Positioning
System (GPS)

4. The basic principle of the Global Positioning
System (GPS)
• A GPS receiver must be locked on to the signal of at least three
satellites to calculate a two-dimensional position (latitude and
longitude) and track movement.
With four or more satellites in view, the receiver can determine the
user’s three-dimensional position (latitude, longitude and altitude).

5. Working Of GPS
● A GPS receiver can tell its own position by using the position data of itself,
and compares that data with 3 or more GPS satellites.
● To get the distance to each satellite, the GPS transmits a signal to each
satellite.
The signal travels at a known speed.
The system measures the time delay between the signal transmission
and signal reception of the GPS signal.
The signals carry information about the satellite’s location.
Determines the position of, and distance to, at least three satellites.
The receiver computes position using TRILATERATION.

6. Trilateration

7. Space Segment
• GPS satellites fly in circular orbits at an altitude of 20,200 km and with a
period of 12 hours.
• Orbital planes are centered on the Earth.
• Each satellite makes two complete orbits each sidereal day.
• It passes over the same location on Earth once each day.
• Orbits are designed so that at the very least, six satellites are always
within line of sight from any location on the planet.

8. Control Segment
• The Control Segment consists of 3 entities:
– Master Control Station
– Monitor Stations
– Ground Antennas
– NGA Monitor Stations
– Air Force Satellite Control Network (AFSCN) Remote
Tracking Stations

9. Strategic Locations

10. Master Control Station
• The master control station, located at Falcon Air Force Base in Colorado
Springs, Colorado, is responsible for overall management of the remote
monitoring and transmission sites.
Performs the primary control segment functions, providing command and
control of the GPS constellation.
Generates and uploads navigation messages and ensures the health and
accuracy of the satellite constellation.
Monitors navigation messages and system integrity, can reposition
satellites to maintain an optimal GPS constellation.
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11. Monitor Stations
• Six monitor stations are located at Falcon Air Force Base in Colorado, Cape
Canaveral, Florida, Hawaii, Ascension Island in the Atlantic Ocean, Diego
Garcia, and in the South Pacific Ocean.
Checks the exact altitude, position, speed, and overall health of the
orbiting satellites.
The control segment uses measurements collected by the monitor
stations to predict the behavior of each satellite's orbit and clock.
The prediction data is up-linked, or transmitted, to the satellites for
transmission back to the users.
The control segment also ensures that the GPS satellite orbits and clocks
remain within acceptable limits.
A station can track up to 11 satellites at a time.
This "check-up" is performed twice a day, by each station.
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12. Ground Antennas
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Ground antennas monitor and track the satellites from horizon to horizon.
They also transmit correction information to individual satellites.
Communicate with the GPS satellites for command and control purposes.
Four dedicated GPS ground antenna sites co-located with the monitor
stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape
Canaveral.

13. AFSCN Remote Tracking
Stations
• Air Force Satellite Control Network (AFSCN) provides support for the
operation, control, and maintenance of a variety of United States
Department of Defense satellites.
This involves continual Tracking, Telemetry, and Command (TT&C).
It also provides prelaunch simulation, launch support, and early orbit
support while satellites are in initial or transfer orbits and require
maneuvering to their final orbit.
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14. NGA Monitor Stations
• The NGA Monitor collects, processes, and distributes GPS observations,
environmental data, and station health information.
It also provides 24/7 data integrity monitoring.
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15. User Segment
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The user's GPS receiver is the User Segment of the GPS system.
GPS receivers are generally composed of an antenna, tuned to the
frequencies transmitted by the satellites, receiver-processors, and a
highly-stable clock (commonly a crystal oscillator).
They include a display for showing location and speed information to the
user.
A receiver is often described by its number of channels this signifies how
many satellites it can monitor simultaneously.
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16. GPS Signals
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Coarse/Acquisition code
Precision code
Navigation message
Almanac
Data updates

17. GPS Frequencies
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L1 (1575.42 MHz)
L2 (1227.60 MHz)
L3 (1381.05 MHz)
L4 (1379.913 MHz)
L5 (1176.45 MHz)

18. Frequency Information
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The C/A code is transmitted on the L1 frequency as a 1.023 MHz signal.
The P(Y)-code is transmitted on both the L1 and L2 frequencies as a
10.23 MHz signal.
L3 is used by the Defense Support Program to signal detection of missile
launches, nuclear detonations, and other applications.
L4 is used for additional correction to the part of the atmosphere that is
ionized by solar radiation.
L5 is used as a civilian safety-of-life (SoL) signal.
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19. Frequency L2C
• Launched in 2005, L2C is civilian GPS signal, designed specifically to meet
commercial needs.
L2C enables ionospheric correction, a technique that boosts accuracy.
Delivers faster signal acquisition, enhanced reliability, and greater
operating range.
L2C broadcasts at a higher effective power making it easier to receive
under trees and even indoors.
It is estimated L2C could generate $5.8 billion in economic productivity
benefits through the year 2030.
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20. Accuracy
• The position calculated by a GPS receiver relies on three accurate
measurements:
– Current time
– Position of the satellite
– Time delay for the signal
The GPS signal in space will provide a "worst case" accuracy of 7.8 meters
at a 95% confidence level.
GPS time is accurate to about 14 nanoseconds.
Higher accuracy is available today by using GPS in combination with
augmentation systems. These enable real-time positioning to within a few
centimeters.
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21. Issues That Affect Accuracy
• Changing Atmospheric Issues:
– Radio signals travel at different velocities through the atmosphere.
– It changes the speed of the GPS signals unpredictably as they pass
through the ionosphere.
– The amount of humidity in the air also has a delaying effect on the
signal.

22. Issues That Affect Accuracy
(cont’d)
• Clock Errors :
– Can occur when a GPS satellite is boosted back into a proper orbit.
– The satellite's atomic clocks experience noise and clock drift errors.
GPS Jamming :
– It limits the effectiveness of the GPS signal.
– GPS jammer is a low cost device to temporarily disable the reception
of the civilian coarse acquisition (C/A) code.
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23. Issues That Affect Accuracy
(cont’d)
• Multi-path Issues :
– The multipath effect is caused by reflection of satellite signals
(radio waves) on objects.
– The reflected signal takes more time to reach the receiver than the
direct signal.

24. Methods of Improving
Accuracy
• Precision monitoring
– Dual Frequency Monitoring
– Carrier-Phase Enhancement (CPGPS)
– Relative Kinematic Positioning (RKP)
• Augmentation

25. A. Dual Frequency Monitoring
• Refers to systems that can compare two or more
signals.
• These two frequencies are affected in two different
ways.
• After monitoring these signals, it’s possible to calculate
what the error is and eliminate it.
• Receivers that have the correct decryption key can
decode the P(Y)-code transmitted on signals to
measure the error.

26. B. Carrier-Phase Enhancement (CPGPS)
• CPGPS uses the L1 carrier wave, which has a period
1000 times smaller than that of the C/A bit period, to
act as an additional clock signal and resolve
uncertainty.
• The phase difference error in the normal GPS amounts
to between 2 and 3 meters (6 to 10 ft) of ambiguity.
• CPGPS works to within 1% of perfect transition to
reduce the error to 3 centimeters (1 inch) of ambiguity.
• By eliminating this source of error, CPGPS coupled with
DGPS normally realizes between 20 and 30 centimeters
(8 to 12 inches) of absolute accuracy.

27. C. Relative Kinematic Positioning (RKP)
• Determination of range signal can be resolved to an
accuracy of less than 10 centimeters (4 in).
• Resolves the number of cycles in which the signal is
transmitted and received by the receiver.
• Accomplished by using a combination of DGPS
correction data, transmitting GPS signal phase
information and ambiguity resolution techniques via
statistical tests — possibly with processing in real-time.

28. • Augmentation
– Relies on external information being integrated into the calculation
process.
– Some augmentation systems transmit additional information about
sources of error.
– Some provide direct measurements of how much the signal was off in
the past
– Another group could provide additional navigational or vehicle
information to be integrated in the calculation process.

29. Augmentation Systems
• Nationwide Differential GPS System (NDGPS)
– Ground-based augmentation system that provides increased accuracy
and integrity of GPS information to users on U.S. land and waterways.
– The system consists of the Maritime Differential GPS System operated
by the U.S. Coast Guard and an inland component funded by the
Department of Transportation.

30. Augmentation Systems (cont’d)
• Wide Area Augmentation System (WAAS)
– Satellite-based augmentation system operated by the Federal Aviation
Administration (FAA), supports aircraft navigation across North
America.
• Global Differential GPS (GDGPS)
– High accuracy GPS augmentation system, developed by the NASA Jet
Propulsion Laboratory (JPL) to support the real-time positioning,
timing, and determination requirements of NASA science missions.
– Future NASA plans include using the Tracking and Data Relay Satellite
System (TDRSS) to transmit via satellite a real-time differential
correction message.

31. Applications
• Civilian
– Geotagging : Applying location coordinates to digital objects such as
photographs and other documents.
– Disaster Relief/Emergency Services
– Vehicle Tracking Systems
– Person Tracking Systems
– GPS Aircraft Tracking
– Telematics: GPS technology integrated with computers and mobile
communications technology in automotive navigation systems.

32. Applications (cont’d)
• Military
– Target Tracking: Tracking potential ground and air targets before
flagging them as hostile.
– Navigation
– Missile and Projectile Guidance: Allows accurate targeting of various
military weapons including cruise missiles and precision-guided
munitions
– Reconnaissance
– Search and Rescue: Downed pilots can be located faster if their
position is known.

33. Applications (cont’d)
• Other Applications
– Railroad Systems
– Recreational Activities
– Weather Prediction
– Skydiving
– And many more!